Thyristor dimmer circuits are frequently used to control incandescent lamps, LED lamps and other loads. FIG. 1A illustrates a thyristor dimmer circuit 100. The thyristor dimmer circuit 100 includes a triac-type thyristor 104 connected to an AC voltage source 112. The thyristor 104 is also connected in series to an inductor 120 and to an incandescent lamp 108. A capacitor 124 is connected in parallel to the thyristor 104 and to the inductor 120. A pulse generator circuit 116 applies a periodic gate pulse to the gate terminal 128 of the thyristor 104. In operation, the thyristor 104 turns ON in response to the gate pulse being applied to the gate terminal 128. The thyristor 104 turns OFF at the zero crossing of the AC voltage or when the current flowing through the thyristor 104 drops below a threshold current, known as a holding current.
FIG. 1B illustrates the AC voltage supplied by the voltage source 112, the gate pulse Vgate, and the output voltage Vout across the incandescent lamp 108. The gate pulse Vgate is applied to the gate terminal at a phase angle typically between 30 and 150 degrees. In response, the thyristor 104 turns ON, resulting in the output voltage Vout being applied to the incandescent lamp 108. Due to the switching of the thyristor 104 at a phase angle near 90 degrees, the output voltage Vout transitions rapidly from 0 volt to Vmax at 90 degrees, and again transitions rapidly from 0 volt to Vmin at 270 degrees. As will be understood, the rapid transitions of Vout generates higher order harmonics causing EMI. The inductor 120 and the capacitor 124 act to suppress the EMI. Also, the incandescent lamp 108 provides resistive damping to the EMI and provides the necessary holding current to the thyristor 104.
If the thyristor circuit 104 is used to feed an active load such as a power converter, instead of the incandescent lamp 108, the power converter will not provide the resistive damping or the holding current. FIG. 2 is a power control circuit 200 having a thyristor 204 for controlled power delivery to a power converter 228. A pulse generator circuit 206 applies a periodic gate pulse to a gate terminal 205 of the thyristor 204. An AC voltage source 208 applies an AC voltage to the thyristor 204. The thyristor 204 generates a modified AC output having rapid transitions as illustrated in FIG. 1B. An input filter 232 comprising capacitors 212, 216 and a common mode choke 220 suppresses EMI generated by the power converter 228. A full bridge rectifier 224 comprising diodes D1-D4 rectifies the modified AC voltage to generate a DC voltage at the rectifier output terminal 236. The rectified voltage at the rectifier output terminal 236 is applied to the power converter 228. A capacitor 240 filters the high-frequency pulsed current drawn by the power converter. The power converter 228 may be a switched mode dc-to-dc converter that generates a controlled DC current at the converter output terminal 244. The controlled DC current is filtered by a converter filter capacitor 248 and is applied to an LED bank 252.
As discussed before, the power converter 228 does not provide the resistive damping to the EMI. Also, the power converter 228 does not instantaneously provide the necessary holding current to the thyristor circuit 204. Consequently, ringing current, shown in FIG. 3, generated by the power converter 228 flows through the thyristor circuit 204, causing the current flowing through thyristor 204 to fall below the holding current. Lacking the holding current, the thyristor 204 turns OFF and remains OFF until the next occurrence of the pulse, resulting in erratic performance of the power converter 228 and flickering at the LED load.